Elastomeric objects are formed by shaping a precurser substance or compound capable of being cured into an elastomeric compound into a desired shaped greenware or green structure and then applying heat and optionally pressure to cure the green structure into an elastomeric object. Most frequently, the elastomeric object being formed is a rubber object and the treatment under heat and optionally pressure to form the rubber object from a formed or "laid up" green structure is frequently referred to as vulcanization. During vulcanization, molecules of the elastomeric substance become cross-linked creating a thermoset elastomer which, where the elastomer is a rubber, is typically possessed of a resilient or rubbery construction. A large number of elastomeric substances are known to be vulcanizable into an elastomeric thermoset material. Such vulcanizable elastomers generally can be classified either: as thermosetting resins, typically plastics; and rubbers, both natural and synthetic.
In the vulcanization of rubbers, typically a vulcanizing compound is employed together with the rubber to assist in cross linking molecules of the rubber. Most often this vulcanizing compound is sulfur, but a variety of other vulcnizing agents are well known. A blend of unvulcanized rubber and these vulcanizing agents may also include accelerators for vulcanization and activators for vulcanization. Suitable vulcanizing accelerators and vulcanizing activators are set forth in the Vanderbilt Rubber Handbook, Babbit, ed. R. T. Vanderbilt Company, Inc., Norwalk, Conn., pp 463-543.
Among activators frequently employed are reaction products of fatty acids and metallic oxides, typically zinc oxide (ZnO.sub.2). The fatty acid employed typically is stearic acid, an acid readily available because of its bountiful occurrence in nature. The use of other fatty acids is known however. It is believed that zinc oxide and fatty acid contained within the green rubber compound react to form a salt of the fatty acid and the zinc. Since the fatty acid contained in green rubber is typically stearic acid, this salt is conveniently termed zinc stearate. Since natural green rubber contains fatty acid and since synthetic green rubber typically contains fatty acid residuals relating to the function of the green synthetic rubber, whether or not fatty acid must be added to green rubber in forming a green rubber structure often is solely a function of the quantity of fatty acid already present in the green rubber.
A number of compounds, when incorporated into a green rubber object, under certain conditions, may produce a bloom on the surface of an object formed from the green rubber compound. Bloom on green rubber is an elevated concentration of a particular additive material on the surface of a green rubber structure in relation to the average concentration of that particular additive material in the rubber matrix of the green structure. Where bloom occurs on green or uncured rubber stocks, the main difficulty that results is a loss of tack. Where coadherence between layers of green rubber laid up in building a green rubber object or structure is important to the integrity of a final vulcanized rubber structure, the loss of tack in building such green objects can result in deformities and uneven vulcanization. The problem of loss of tack associated with additive bloom is particularly critical in the manufacture of tires, where any resulting tire deformities can provoke an unsafe driving condition.
Particularly, where zinc and fatty acid compounds are being employed as activators for vulcanization, the appearance of a zinc-fatty acid reaction compound bloom can result in the manufacture of a rubber article wherein co-adherence of portions of the rubber article separately laid up during preparation of the precurser green object are not properly co-adhered. Where bloom developes in a green rubber compound that includes a zinc-fatty acid reaction product, two separate phenomenon have been frequently observed.
In one phenomenon, the zinc appears to bloom as a salt of the fatty acid and the bloom appears within a matter of seconds or minutes following formation of the green rubber into a shaped object. For purposes of this specification designated as thermal bloom, this relatively rapid blooming of the salt of the zinc and fatty acid, herein termed zinc stearate for convenience, appears to be linked to situations wherein the last mastication of the green rubber including the zinc and any fatty acid additives occurred above the melting point of the reaction product salt of the zinc and the fatty acid. Careful control of the temperature of the last mastication of the green rubber before forming into a green rubber object can prevent or suppress zinc stearate thermobloom with reliable predictability.
A second form of zinc stearate bloom occurs after a green rubber object has aged for a period of time. Aging bloom typically developes over a more extended time period and the development of aging bloom often appears to be enhanced by exposure of the green rubber to conditions of elevated humidity and temperature. Where aging bloom occurs only sporadically in a few of many batches of green rubber produced for incorporation into green rubber objects, an inability to predict these occurrences of aging bloom can result in significant quantities of such green rubber being wasted or scrapped. Were a reliable means for assuring that zinc stearate aging bloom was suppressed in the making and storing of green rubber and green rubber objects, significant economies could be achieved in reducing defectively manufactured product, excessive recycle of green rubber, and waste associated with the production of unusable green rubber material.